Stressed fabric structures conventionally have walls and a roof formed of fabric secured to and extending under tension between spaced arched beams supported on a base. Such structures are extremely easy to assemble and disassemble, and hence are highly portable. Because the fabric is maintained in a stressed state when the structure is erected, such structures can readily withstand high winds and other forms of external loading such as snow and rain. Thus, they have become very popular for many applications, including temporary or permanent warehouses, enclosures for athletic facilities such as hockey rinks, temporary exhibition halls, military storage areas, hangars, etcetera. Such structures are described and illustrated for example in my earlier U.S. patents namely; U.S. Pat. No. 4,118,904 issued Oct. 10, 1978, entitled "Building Structure", U.S. Pat. No. 4,137,687 issued Feb. 6, 1978, entitled "Stressed Membrane Space Enclosure", and U.S. Pat. No. 3,780,477 issued Dec. 25, 1973, entitled "Demountable Building".
One of the technical problems presented by such structures is providing a suitable doorway for entrance to or exit from the structure. Because the walls are formed from fabric stretched between arched beams, a conventional frame may be provided between adjacent pairs of arched beams to support a vertically hinged door.
Where the structure is used, for example, as a hangar or warehouse, a larger doorway may be required. To accomplish this, in the past, a series of arched beams have been provided at the end of the structure, these arched beams being pivotably movable in sequence as a power cable connected to a last of the arched beams was retracted towards the building, to open the entire end of the structure (like a portion of a "clam shell"). A problem with this construction has been that when raising or lowering the door, the cable may break and the arches collapse, damaging or destroying anything underneath them. In addition, it is extremely difficult to raise or lower such a door in heavy wind storms using a cable, since the door can fly completely open as soon as the tie-down bars, securing the lowermost arch in position, are released. As a result, the size of the arched beams and their retracting mechanisms, as well as the concrete footings used to hold them in position, must be of significant size and weight.
It is an object therefore of the present invention to provide an alternative construction of door system for stressed fabric structures that avoids many of the problems of such prior known systems and that can be safely raised or lowered in a variety of conditions.